This invention relates to an apparatus for maintaining constant tension on a web of thermoplastic material being processed so that bags of consistent width may be produced in a bag making apparatus which is capable of producing bags of different widths.
A wide variety of plastic bag products are commercially available today to enable the consumer to package and protect a variety of food products. Such bags are available in sizes varying from pint size to larger quart, gallon, and jumbo sizes.
Continuous, rotary bag making machines are known in the art. Typically, these machines include a rotating drum having a plurality of sever and seal stations equally spaced about the periphery of the drum. A continuous web of thermoplastic material, folded upon itself in the machine direction to form a "U-folded" web, is fed continuously onto the drum and then to the sever and seal stations.
Generally, clamping assemblies are moved into position over individual sever and seal stations and against the drum periphery to hold the web in position as the drum rotates. During the rotation of the drum, heated wires, carried in recesses within the drum, are caused to move outwardly to sever and seal the web, forming side seams on an individual bag. Typically, a plurality of heated wires are contained within the drum, with one wire being associated with each sever and seal station.
Such bag making machines produce bags which are as deep as the folded web and which are as wide as the spacing between sever and seal stations. In such machines while the depth of the bags may be readily regulated by cutting the web to width prior to folding, changing the finished bag width is much more difficult. In those apparatuses, it is not possible to change the width of the bags except by severing only at every second or third station around the drum. Consequently separate machines are required to make each individual size of bag, and machines cannot readily be converted to produce bags of a size different than the machine was originally designed to produce.
Other bag making machines, such as those taught by Achelpohl et al, U.S. Pat. Nos. 4,331,502 and 4,115,183, utilize tucking mechanisms to increase production rates on the machines. In these machines, the sever and seal stations are located closely about the periphery of the drum, and a tucking mechanism acts to push the web inwardly into a groove within the periphery of the drum. The depth of the tuck determines the width of the individual bags, with a greater depth of tuck producing a bag of greater width.
Again, however, such machines with tucking mechanisms cannot be easily modified to make bags of a width different than that the machine was designed to do. That is, the tuck mechanisms on such machines are operated by cams which are designed to push the web into a groove a predetermined distance. Such cams may be modified only by disassembling them and recutting the cam controlling the tucking mechanism, or by replacing one cam size with another. Such operations are not only difficult but costly in both labor, expense, and downtime of the machines. Moreover, because the tucking operation places stretching forces on the plastic web, the consistency of width of the bags produced by such machines is not good.
More recently, bag making machines have been developed which utilize an outwardly directed tucking mechanism to control the depth of tuck. For example, Savich, U.S. Pat. No. 4,557,713 and Savich et al, U.S. Pat. No. 4.609,367, teach bag making machines which have an outward tucking cam mechanism. A cam, which is adjustable for controlling the depth of tuck, is moved in a radial direction to control the tuck depth, and thus, the bag width. Production of different sized bags on such machines is possible.
However, even these more recent machines suffer from problems in achieving bags having a consistent width. Because of the tucks taken in the web, the overall length of the web on the rotating drum is greater than that being fed from the drive rolls at the web source. This requires that the drive rolls be driven faster than the speed of rotation of the drum. Because these machines are typically operated at speeds approaching 300 feet per minute, multiple tucks per second must be made on the drum.
The ability of the speed of the drive rolls to be regulated in the fraction of a second in which a tuck is made is beyond the capabilities of these machines. Thus, the web itself is subjected to instantaneous stretching and acceleration forces several times a second. Applicants have found that operation of the drive rolls at a speed greater than the speed of rotation of the drum alone may be insufficient to avoid such instantaneous stretching forces, particularly in those instances where relatively large tucks must be taken in the film to produce larger sized bags.
Because of these forces, it becomes extremely difficult to control accurately the width of each individual bag as stretching and slippage occur on the rotating drum. Accordingly, there is still a need in the art for a film tension compensation device which maintains constant tension on a film web in a bag making machine which is capable of making bags of differing widths.